"I can't taste it" and "it dissolved more quickly than I expected" doesn't make for particularly great evidence to support this grand sugar replacement conspiracy.

You might have recently had a mild cold/flu/covid infection that left you with a diminished sense of taste that day.

The time it takes sugar to dissolve is affected by a lot of things, including the size of the granules, which might have been closer to powdered sugar than the much larger crystals you're used to seeing in restaurant packets. Just because your drink was cold doesn't mean sugar can't very quickly dissolve in it.

If you're genuinely concerned about this, get a sample of it and send it to a chem lab for testing. That's going to be the easiest way to get evidence worthy of the claim you're making ("it isn't sugar").